(Excuse me for my bad English).
This column labeled "WBS" let you index the stucture of the "tree".
For example, for a specific objective called "1_SO1" ("1" stands for the value in "WBS" cell and "SO1" is the specific objective's name) you can subordinate the tasks labeled "1.1_Task 1" and "1.2_Task 2" by typing the name on the column called "Name" and set the hierarchy of the task by using the "tab" on the keyboard (just like the use of "schemes" in a text processor).
Note that by default configuration every new task added is indexed as the previous one, i.e. if a task is indexed "3.2" the next added will be indexed as "3.3".
I'm started using this tool today, I still don't know how to "draw" the WBS in Gantter.
Hope this is useful for you.
Regards from Uruguay, South America!